The field of tribology deals with the science of interacting surfaces in relative motion. Tribology generally involves the study of friction, wear, and lubrication in relation to such surfaces. Tribological materials are generally characterized by a variety of parameters including, inter alia, wear, load and velocity carrying capacity, coefficient of friction, coefficient of expansion, stiffness, and dimensional stability.
Early tribological materials used in applications where wear resistance and low friction was desired in sliding interfaces were generally metal such as brass, bronze, and other metal alloys, and woods, especially hard woods. The limitations of these materials for friction and wear applications are well known and include the need for constant lubrication, heavy weight, rapid wear, high expense of fabrication, and other problems. These problems drove the development of plastic tribological materials for bearing applications, which to a certain extent addressed some of these limitations.
Plastic bearings are generally made by incorporating additives such as fillers, reinforcement materials, and/or solid lubricants to a polymeric material. The tribological and other properties of such materials depend on the particular polymeric matrix utilized as well as the particular fillers, reinforcements and lubricants compounded with the polymeric matrix material.
Plastic bearings have replaced other materials in many applications because they have high weight to strength ratios and can be made self-lubricating, among other desirable characteristics. Although plastic bearings are important in many applications, their use has been limited in some instances. For example, the use of plastic bearings in high performance applications involving high loads or high velocities has been limited because under such extreme conditions of load or velocity, plastic bearings are generally prone to failure due to the high frictional heat generated. The high frictional heat generated causes softening and melting of the polymeric matrix material. In addition, there are many applications in which plastic bearings generate an unpleasant squeal, as well as excessive heat.
The “wear” of a material generally refers to the amount of material removed from a bearing surface as a result of the relative motion of the bearing surface against a surface with which the bearing surface interacts. The wear of a material is generally reported as a “wear factor” or “K-factor.” As a relative measure of the performance of materials under the same operating conditions, K-factors have proven to be highly reliable.
The load and velocity bearing capability of a material is generally considered that combination of load and speed at which the coefficient of friction or the temperature of a bearing surface fails to stabilize. As used herein, the term “PV limit” will be used to denote the pressure-velocity relationship determined by the combination of load and speed at which the coefficient of friction or the temperature of a bearing surface fails to stabilize, expressed by the product of the unit pressure P (psi) based upon the projected bearing area and the linear shaft velocity V (FPM).
Any improvement in the tribological properties of plastic bearing is desirable.